1. Field of the Invention
The present invention relates to a method and to a device for reducing the effect of the vibration generated by the driveline of a helicopter, as well as to a helicopter equipped with such a device.
2. Background Art
In the known way, the driveline of a helicopter includes at least:
a main rotor providing lift and forward travel; PA1 as appropriate, a counter-torque tail rotor; PA1 at least one engine providing the motive power needed to drive the said main and tail rotors; and PA1 transmission boxes and associated shafts. PA1 the frequency of the first torsional mode of the driveline is close to the control frequencies, for which controlling the engine gives high gain; and PA1 the rotor lacks damping for the movement of the blades in phase because the drag adapter acts in this case only on the differential movement of the adjacent blades. PA1 the values of at least one parameter representing the effect of the vibration generated by the said driveline are measured; PA1 from the measured values of the said parameter, a control signal for the collective pitch of the blades of the said rotor is formed, the said control signal being such that applying the said control signal to the system for controlling the collective pitch of the blades of the said rotor generates a set of forces and of moments which oppose the said effect of the vibration generated by the driveline; and PA1 either to the main rotor providing lift and forward travel of the said helicopter, PA1 or, as appropriate, to the counter-torque tail rotor of the said helicopter. PA1 the speed, acceleration or the rotation of a shaft of the driveline; PA1 the torque exerted on a shaft of the driveline; PA1 a stress, an acceleration, a speed or a displacement generated on the fuselage of the helicopter; PA1 a parameter of the engine; or PA1 an environmental parameter, especially temperature and altitude. PA1 at least one sensor capable of measuring the values of at least one parameter representing the effect of the vibration generated by the said driveline; and PA1 computation means capable: PA1 on the driveline of the helicopter in order to measure, for example, one of the following parameters: the speed, the acceleration or the rotation of a shaft of the driveline or a torque exerted on a shaft of the said driveline; and/or PA1 on the fuselage of the helicopter, in order to measure, for example, one of the following parameters: a stress, an acceleration, a speed or a displacement generated on the said fuselage; and/or PA1 on the engine of the helicopter.
It is known that under certain flight conditions the natural modes of the said driveline may become coupled with those of the fuselage, especially as the rotor speed varies and as a result of the yawing movement of the fuselage, in such a way as to generate resonance phenomena capable of causing substantial vibration of the driveline. Such vibration is capable of making the assembly thus coupled unstable or difficult to fly.
In the known way, the said coupling may especially be brought about by the controlling of the engine or by a lack of damping at the rotor, or even under the influence of the navigation system such as the automatic pilot or, as appropriate, the helicopter electrical or optical flight controls.
It is also known that the vibration which is the most troublesome as regards stabilizing the driveline and flying the helicopter is that due to the first torsional mode of the driveline, for which mode the movement of the blades of the main rotor is in phase opposition with that of the main transmission box of the driveline.
To overcome these drawbacks, a known solution recommends, naturally in addition to damping the rotor, reducing the gain in the engine control.
This solution is not, however, satisfactory, particularly for high-tonnage helicopters because during flight it is often necessary to have high gain in the engine control, especially in order to obtain good pickup.
Furthermore, it will be noted that the effect of the aforementioned troublesome vibration is generally greater when the helicopter rotors are equipped with so-called "inter-blade" drag adapters corresponding to blade dampers mounted, not between each blade and the rotor mast, but directly between two adjacent blades. This is because in this case: